An 18-year-old man was arrested by British police in the city of Dover in connection with Friday's rush hout "terrorist" bombing of a London subway train, calling the arrest “significant” but warned that the country’s threat level remained at its highest, "critical" point.

“We have made a significant arrest in our investigation this morning. Although we are pleased with the progress made, this investigation continues and the threat level remains at critical,” Deputy Assistant Commissioner Neil Basu, Senior National Coordinator for Counter Terrorism Policing, said.

It was unclear if the man arrested was the person believed responsible for the attack, which has been classified as terrorism. Late on Friday, the Islamic State claimed responsibility for the improvised bomb attack via its news agency, Amaq, saying it was carried out by a “detachment.” There was no independent indication of the group’s role. In another separate statement, Islamic State said it had deployed several explosives but only one detonated. That too couldn’t be independently verified.

The man was arrested in the port area of the city, a major hub for ferry transport between the U.K. and France. Authorities have been urgently tracking down leads in the attack, searching through security camera footage and other information.